1. Field of the Invention
The present invention relates to an electrolyte material for a polymer electrolyte fuel cell, a liquid composition comprising the electrolyte material, and a membrane/electrode assembly for a polymer electrolyte fuel cell containing the electrolyte material in a catalyst layer.
2. Discussion of Background
As an electrolyte material contained in a catalyst layer of a membrane/electrode assembly for a polymer electrolyte fuel cell, the following polymer (1) has been known.
A polymer (1) having sulfonic acid groups (—SO3H groups) converted from —SO2F groups in a polymer having repeating units based on a compound represented by the following formula (m3) and repeating units based on tetrafluoroethylene (hereinafter referred to as TFE):CF2═CF(OCF2CFZ)mOp(CF2)nSO2F   (m3)wherein Z is a fluorine atom or a trifluoromethyl group, m is an integer of from 0 to 3, p is 0 or 1, and n is from 1 to 12, provided that m+p>0.
A polymer electrolyte fuel cell is required to be operated under low humidity conditions where the relative humidity of the reaction gas (fuel gas and oxidant gas) is low, and even under no humidity conditions, in order to simplify the fuel cell system or to reduce the cost. In order to obtain sufficient power generation characteristics (such as output voltage) under low or no humidity conditions, it is required to increase the proton conductivity of the electrolyte material contained in the catalyst layer, i.e. to increase the ion exchange capacity of the electrolyte material.
However, if the ion exchange capacity of the polymer (1) is increased, that is, the proportion of repeating units based on the compound represented by the formula (m3) is increased, the water content (water absorptivity) of the polymer (1) is increased, and accordingly, under high humidity conditions where the relative humidity of the reaction gas is high, clogging of pores (flooding) by condensation of water vapor is likely to occur in the catalyst layer. Therefore, it is difficult to increase the ion exchange capacity.
On the other hand, as the electrolyte material, a polymer having a cyclic structure in its molecule has been known. It may, for example, be a polymer (2) having sulfonic acid groups (—SO3H groups) converted from —SO2F groups in a polymer having repeating units based on a perfluoromonomer having a —SO2F group and a dioxolane ring and repeating units based on a perfluoromonomer having no —SO2F group and having a dioxolane ring (Patent Document 1).
A polymer electrolyte fuel cell employing the polymer (2) as the electrolyte material of the catalyst layer is characterized in that it is excellent in the power generation characteristics as compared with one employing the polymer (1). In order to obtain higher performance under low or no humidity conditions, it is effective to increase the ion exchange capacity of the polymer (2). However, by merely increasing the ion exchange capacity of the polymer (2), the water content (water absorptivity) of the polymer (2) is drastically increased. Accordingly, under high humidity conditions where the relative humidity of the reaction gas is high, flooding is likely to occur, whereby the power generation characteristics (such as output voltage) tend to be decreased, and no sufficient characteristics of the polymer (2) can be obtained.
Patent Document 1: WO2004/097851